fix: make workspace panels self contained

docs/PRODUCT_SPEC.md

@@ -66,15 +66,14 @@ surface change never silently launches a process, moves the pane to another
 host, or changes which agent owns the session. Unsupported surfaces are shown as
 unavailable rather than simulated.
 
-The default workspace has three horizontal bands: the top 20% shows context,
-pane/agent state, tools, and goal; the middle 60% is a responsive set of vertical
-work lanes; and the bottom 20% is the decision shelf. The work band shows one
-lane in compact windows, three at normal desktop widths, and up to five on an
-ultrawide display. One lane owns keyboard input at a time, and selecting a lane
-never pauses the others. Explicit split trees and dashboard layouts remain
-supported, but they do not displace this terminal-first default.
+The default workspace is a responsive row of self-contained vertical panels:
+one panel in compact windows, three at normal desktop widths, and up to five on
+an ultrawide display. Every panel has its own 20% context region, 60% terminal or
+work surface, and 20% decision shelf. One panel owns keyboard input at a time,
+and selecting it never pauses the others. Explicit split trees and dashboard
+layouts remain supported, but they do not displace this cmux-style default.
 
-An optional decision shelf sits below the active work surface. It may show an
+An optional decision shelf sits below each pane's work surface. It may show an
 answer draft, two or three concrete choices, why each was suggested, and the
 capability each choice would need. Suggestions are inert data. Choosing one may
 prepare a typed workspace plan, but any command, file mutation, credential use,